(1) Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly to a method for fabricating a gate electrode structure of a compound semiconductor device.
(2) Description of the Related Art
Conventionally, for forming an electrode structure of a semiconductor device, especially for forming a heat resistance gate electrode of a GaAs FET, the film formation is carried out, as shown in FIG. 1A as a first prior art example, such that an operating layer (n-GaAs) 12 is first formed on a GaAs substrate 11 by an epitaxial growth process, etc. and then, as shown in FIG. 1B, a refractory metal layer or a refractory metal compound layer 31 is deposited by a vapor deposition process, a sputtering process, etc. On the resulting surface, a titanium nitride layer 32 and also a gold layer 16 are sequentially deposited by a vapor deposition process, a sputtering process, etc. and then, as shown in FIG. 1C, the gold layer 16 is processed by an ion milling using a photoresist 15 as a mask, and further the titanium nitride layer 32 and the refractory metal (or compound) 31 are processed by a reactive ion etching (RIE) process using the gold layer 16 as a mask, whereby a gate electrode structure is formed (FIG. 1D).
The gold layer 16 is formed in order to reduce the gate resistance, and the titanium nitride layer 32 is formed in order to enhance the adhering properties of the refractory metal (or compound) 31 and the gold layer 16 and, during the thermal treatment at about 500.degree. C., to act as a barrier metal to prevent the gold 16 from transmitting through the refractory metal (or compound) 31 to be diffused to the GaAs substrate 11 and to deteriorate Schottky characteristics (Japanese Patent Application Kokai Publication No. Sho 63-51679).
Also, in a second prior art example, after a silicon oxide film 21 is grown over the GaAs substrate 11 having the operating layer (n-GaAs) 12 as shown in FIG. 2A, an opening is formed in the silicon oxide film 21 by a reactive ion etching process using a photoresist as a mask. Then, as shown in FIG. 2B, formed on the resulting entire surface are a refractory metal (or compound) layer 31, a titanium nitride layer 32 and a platinum layer 41 by a vapor deposition process, a sputtering process, etc. Next, as shown in FIG. 2C, after a gold layer 16 is formed by a plating process using a photoresist 15 as a mask and a platinum 41 as plating pass, the platinum layer 41 is processed by an ion milling process, and the titanium nitride layer 32 and the refractory metal (or compound) layer 31 are processed by a reactive ion etching by using a gold layer 16 as a mask (FIG. 2D).
However, in the multi-layer structure composed of the refractory metal (or compound) layer, the titanium nitride layer and the gold layer as in the above first prior art example, a sputter etching step using an argon ion gas is required after the formation of the refractory metal (or compound) layer in order to enhance the adhering property of the titanium nitride and to clean the surface of the refractory metal (or compound) layer and, in the process of forming the electrode after the formation of metal films, an ion milling process is required for processing the gold and a reactive ion etching process is required for processing the titanium nitride and the refractory metal (or compound) layer, which results in an increase in the number of steps in the process. Also, since the ion milling is used, fine metal chips produced during the milling tend to adhere to peripheral portions of the electrode thereby causing the occurrence of electric short-circuiting and operational failure.
Also, in the second prior art example, due to an advancement of the miniaturization of a gate electrode, it makes it difficult, after the formation of the refractory metal (or compound) layer, to sufficiently fill inside the gate with the titanium nitride, thereby reducing the function thereof as a barrier metal and reducing the reliability of the gate electrode.